1. Field of the Invention
The present invention relates to a method for separating and concentrating gallium or indium from gallium- and indium-containing solutions.
2. Description of the Related Art
Gallium, which is a metal element obtained in trace amounts as a byproduct of zinc or aluminum smelting, is widely used in compound semiconductors. In the field of compound semiconductors, high purity gallium purified to 6N (99.9999%) or higher is used in the production of GaAs, and GaP, which are, in turn, used for light-emitting diodes, ICs, LSIs, and the like. Similar to gallium, indium is a metal element obtained in trace amounts as a byproduct of zinc or aluminum smelting and mostly used as ITO to form transparent electrode films for liquid crystals.
In conventional practice, ion exchange, solvent extraction, and other techniques are used to selectively separate gallium and indium from solutions containing traces of gallium and indium, and to concentrate these elements. For example, the method disclosed in Japanese Unexamined Patent Application (Kokai) 59-193230 is known as such an ion-exchange technique. According to this technique, a solution containing traces of gallium and indium is passed through a layer of chelating ion-exchange resin under an appropriate pH, the gallium and indium are selectively adsorbed, and these elements are then eluted using a mineral acid.
The following method is also well known as solvent extraction technique: a carboxylic acid-based or phosphoric acid-based chelate extraction chemical is added to an organic solvent, the pH of the aqueous phase is adjusted, and the product is brought into close contact with the aforementioned organic solvent, whereby the gallium and indium in the aqueous phase are selectively extracted as chelates into the organic phase.
The above-described ion-exchange technique, however, requires resin columns and other bulky equipment, irrespective of the recovery volume of gallium and indium. This technique is also disadvantageous in that when large amounts of iron, aluminum, and other impurities are present, failure to remove them in advance will lower the removal efficiency of the resin, block the resin column, and the like.
Solvent extraction is disadvantageous in that large amounts of organic chelating agents and organic solvents are needed for the reactions, so high running costs are incurred and explosion-proof equipment must be used because of safety considerations, resulting in much higher costs in terms of initial investment.
Thus, all these conventional methods are difficult to integrate into future industries in terms of cost, and recovery of trace amounts of gallium and indium at minimal cost is desired.